Corona protection screen for inductor coils in vacuum furnaces



Aug- 1964 l. BECKIUS ETAL v 3,145,300

CORONA PROTECTION SCREEN FOR INDUCTOR v COIL S IN VACUUM FURNACES Filed Sept. 19, 1960 m H WWW 1 Maw: vs,

United States Patent 3,146,360 PROTECTION SCREEN FOR INDUCTOR COILS 1N VACUUM FURNACES Ivar Beckius and Mats Rydinger, Vasteras, Sweden, assignors to Allmiinna Svenslm Eiektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation Filed Sept. 19, 1960, Ser. No. 56,999 Claims priority, application Sweden Sept. 18,

6 Claims. (Cl. 174--12'l) CORONA In vacuum furnaces in which the heating is brought about by means of an inductor coil, it is extremely difficult to prevent flashover between different coil parts and between the coil and surrounding parts of the furnace. The reason that particular problems regarding occurring flashover are present in vacuum furnaces is that the rarified atmosphere has poor insulation properties, and therefore the corona, i.e., electrical gas discharges which can develop into flashover, easily occurs.

In order to prevent corona on the part subjected to an electrical tension, namely the furnace coil and connection leads in the vacuum chamber, these parts are usually first insulated and subsequently provided with a corona protection screen which may consist of a number of conducting layers insulated from each other within the coil, but which are connected together outside the coil and are usually connected to earth or apotential lower than the critical electrical tension at which corona appears.

If rupture arises in the connection between the layers or between a layer and earth potential or other potential under the critical tension where corona appears, for example because of mechanical damage or thermal strain, the layer receives approximately the same potential as that part of the copper conductor which the screen encloses. If rupture arises within a layer, in a similar manner, that part of the layer which after the rupture is not in connection with the other layers or with earth or another potential under the corona limit tension, receives approximately the same potential as the copper conductors situated within this part. Such ruptures result immediately in flashover phenomena so that the furnace can become unusable and the processes which the furnace is intended to carry out must be uncontrollably cut off. Apart from the necessity of re-insulating the furnace coils and possibly replacing or repairing other damaged parts, sometimes the material which is being processed in the furnace may have to be discarded, with the considerable economic losses which this entails.

The present invention relates to a corona protection screen in which flashover phenomena cannot arise as a direct result of a connection between any of its parts or between these parts and the earth potential or another potential lower than the critical electrical tension at which corona appears. The object of the invention is a corona protection screen consisting of layers of conducting or semiconducting material, which are covered on the outside by a tight and pore-free coating of an insulating material, for example silicone rubber. It is advantageous if this material has resistance against corona. The conducting or semiconducting layers are suitably attached to the insulation of the coil with a thermosetting resin. The expression thermosetting resin is intended not only to include resins which are cured by heating but also those resins which cure at room temperature such as completely polymerizable resins. These conducting or semiconducting layers, further, can suitably be made up of metal, sprayed on, or varnishes containing suspended metal particles, in which case the spraying on of the metal or application of the varnish takes place while the resin is still uncured, so that a close connection between the insulation and the said layers is obtained. The resin is then transferred to cured condition, preferably before the consulating covering.

' coil, an epoxy resin 7 suitable.

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ducting of semiconducting layers are embedded in the in- If the connection between the layers or with the earth should be broken and corona arise, the furnace will not be incapable of function since the covering is capable of resisting the strain arising so that flashover does not occur. Consequently, it is possible under these circumstances to continue the process taking place under the planned conditions and thereby obtain the result aimed at with the treatment in the furnace. The furnace need not immediately afterwards be taken out of operation, but it may still be used in the normal way, so that repair of the corona protection screen may be made at a suitable time. Damage of such a nature that corona arises can further be established through the observable corona phenomenon on the coating, which makes it possible to stop the process under complete control, if it is considered This may be the case, for example, if certain parts of the furnace are made of materials which are easily damaged by corona, or if for special reasons it is suspected that the risk of fiashover might be present with continued operation. Because a fault arising can be established before flashover takes place, damages resulting from such a phenomenon consequently can be avoided.

The invention will be described more closely with reference to the FIGURES 1 and 2.

FIGURE 1 shows a side view of a part of a helically wound, water-cooled coil for an induction furnace. The coil is insulated and provided with a corona protection screen according to the invention. In relation to the diameter of the coil, the diameters of the conductor sections are enlarged.

FIGURE 2 shows a section A A of a turn of the coil in FIGURE 1. The coil shown in the figures is only chosen as an example of a coil which may be provided with a corona protection screen according to the invention. The shown corona protection screen constitutes an embodiment of the invention chosen by way of example. FIGURE 3 is a cross-section on the line 13 43 of FIGURE 2.

FIGURE 1 shows which can be seen in more detail in FIGURE 2, and also how the conducting or semiconducting layers insulated from each other within the coil are connected together and to earth outside the coil by leads 2. The conductor consists of a copper pipe 3 which is internally cooled by water. Outside the real insulation 4, which consists for example of a silicone varnish, an epoxy resin, an alkyd varnish, a phenolic varnish, an unsaturated polyester resin, or silicone rubber, is applied a ground layer 5 for the conducting or semiconducting layers 6, 7 and 8, which together with the covering 9 of a tight, porefree and corona-resistant material, form the corona protection screen. Outermost is applied an insulating material 10 for protection against mechanical damages. The insulation 4 may also consist, e.g., of a wrapping of glass fibre tape impregnated with the mentioned resins and varnishes or of a wrapping of silicone rubber tape reinforced with a glass fibre tape. The ground layer 5 consists in the exemplified case of a Wrapping, e.g., with half-overlap of a woven glass fibre tape, e.g., with a width of 19 mm. and a thickness of 0.08 mm. After the glass tape has been applied to the insulation of the is supplied, for example the epoxy resin Araldite XV with Curing Agent XV from Ciba AG., which components are previously mixed in the proportion of parts by weight of the first mentioned and 30 parts by weight of the last mentioned. The mentioned epoxy resin is of the bisphenol type and dissolved in a solvent, and the curing agent is of the anhydride type and also dissolved in a solvent. Heatingcuring as well as room-temperature-curing resin may be used. Instead of the exemplified epoxy resin other num and silver for example layer 7 encloses in this example, as is clear from FIGURE 2,

tion 4. The metal layers are connected to each other E 2 is seen particuvia the connections 13 and 14 and the leads 2 are connected With each other outside the conductor. It is also seen that they are insulated from each other inside the coil. The conductripheral direction by the insulating material 5 present between the end parts of the conducting layers 7 and 8 in the same periphery of the coil. As is seen from FIGURE 2, the insulation 11 is present also between the tWo end parts of the conducting layer 6 at different tape containing a reinforced glass tape, for example of Westinghouse Silastic Tape 9495-12 with a width of 19 mm. and a thickness of 0.25 mm. from Westinghouse Corp. The silicone rubber tape a thickness of plied With half overlap.

sufiicient resistance against tection may be dispensed the pp a n his tape may, e.g., be ap- If the covering 9 itself has mechanical damage, the pro- With. The resin used during of the metal layers is suitably cured after the layers have been sprayed on, but curing may also be deferred until the layers are covered with the tight and pore-free material.

We claim:

1. A vacuum furnace inductor coil adapted to be arranged in a Vacuum chamber, said other, and a tight, pore-free cover of silicone rubber enclosing said electrically conducting parts.

2. A vacuum furnace inductor coil as claimed in claim 1, in which said electrically conducting parts comprise metal layers.

3. A vacuum furnace 1, in which inductor coil as claimed in claim said electrically conducting parts comprise rubber tape.

6. A vacuum furnace inductor coil adapted to be arranged in a vacuum chamber, said mductor coil comprising an electric OTHER REFERENCES Electrical Manufacturing, October 1951, pages 134 7 

1. A VACUUM FURNACE INDUCTOR COIL ADAPTED TO BE ARRANGED IN A VACUUM CHAMBER, SAID INDUCTOR COIL COMPRISING AN ELECTRIC CONDUCTOR FORMING A COIL FOR INDUCTION HEATING, A LAYER OF ELECTRICAL INSULATION APPLIED AROUND SAID CONDUCTOR, A CORONA PROTECTION SCREEN APPLIED AROUND SAID INSULATION ALONG THE ENTIRE LENGTH OF THE CONDUCTOR AND CONNECTED TO A POTENTIAL LOWER THAN THE CRITICAL TENSION AT WHICH CORONA APPEARS, SAID CORONA PROTECTION SCREEN COMPRISING AT LEAST TWO ELECTRICALLY CONDUCTING PARTS EXTENDING IN THE LONGITUDINAL DIRECTION OF SAID CONDUCTOR, ARRANGED SUCCESSIVELY ONE AFTER THE OTHER IN THE LONGITUDINAL DIRECTION OF THE CONDUCTOR AND INSULATED FROM AND PARTIALLY OVERLAPPING EACH OTHER, AND A TIGHT, PORE-FREE COVER OF SILICONE RUBBER ENCLOSING SAID ELECTRICALLY CONDUCTING PARTS. 